Stockholders' Equity - Schedule of Reserve Share of Common Stock for Issuance (Details) - shares	Dec. 31, 2018	Dec. 31, 2017	Jul. 31, 2015
Class of Stock [Line Items]
Shares of common stock reserved for issuance (in shares)	34,960,000	34,425,000
Outstanding stock options
Class of Stock [Line Items]
Shares of common stock reserved for issuance (in shares)	13,590,000	16,268,000
Restricted Stock Units (RSUs)
Class of Stock [Line Items]
Shares of common stock reserved for issuance (in shares)	4,182,000	5,330,000
2015 Equity Incentive Plan
Class of Stock [Line Items]
Shares of common stock reserved for issuance (in shares)	11,986,000	8,857,000	11,400,000
2015 Employee Stock Purchase Plan
Class of Stock [Line Items]
Shares of common stock reserved for issuance (in shares)	5,202,000	3,970,000